Tuning In to Other Worlds

Jupiter, Saturn, Uranus, and Neptune are four planets in our solar system that experience visual light auroras like the one shown above.Image Credit: Michigan Tech

As the Aurora Borealis illuminates the night with sheets of shimmering color, the phenomenon seems silent, almost stealthy. The Aurora isn’t silent, however. There is a radio counterpart that sings a tune for the lights to dance to.

It’s not a song we could hear without a radio receiver, of course. And the electromagnetic radio waves are sent out into space rather than down towards the ground. The radio waves are generated by the Earth’s magnetosphere, an invisible system of magnetic fields, electric currents and charged particles that surrounds the Earth. Sub-atomic particles from the Sun – called "solar wind" – continually hit the Earth’s magnetosphere and create a build-up of energy. Under certain conditions, this stored energy combines with direct energy from the solar wind, energizing electrons and ions inside the magnetosphere. Auroras, or "Northern Lights," are created when some of these energized particles fall onto the atmosphere at high latitudes, generating colored light when they interact with gases. Energy from other particles turns into the low frequency radio waves.

Other planets in our solar system with magnetospheres – Jupiter, Saturn, Uranus and Neptune – also experience visual light Auroras and the related radio emissions. Some scientists think it may be possible to detect planets beyond our solar system by looking for similar radio signals.

A team of scientists working on a radio telescope called the Low Frequency Array (LOFAR) plan to do just that. LOFAR is a joint project of the Naval Research Laboratory (NRL), MIT’s Haystack Observatory, and the Netherlands Foundation for Research in Astronomy. LOFAR is still in the planning stages – the array will not be operational until 2006 or later – but scientists from the NRL, NASA’s Goddard Space Flight Center, the National Radio Astronomy Organization, and the Observatoire de Paris have been testing the feasibility of radio planet detection by using the Very Large Array (VLA) in New Mexico.

The VLA was never designed to detect frequencies below 100 megahertz (MHz), but the scientists have been able to push detection levels down to 74 MHz.

"A highly magnetized planet could have emissions at frequencies such as 74 MHz," says Robert Mutel, professor of astronomy at the University of Iowa. "Below this frequency, it is extremely difficult to conduct sensitive array observations."

But if radio emissions from extrasolar planets are anything like those of our own solar system, the scientists will have to look for much lower frequencies. The Aurora-related radio emissions from Earth, Saturn, Uranus and Neptune are all below 1 MHz. Such bursts from Jupiter occur at frequencies up to 40 MHz. The strength of a planet’s magnetic field determines the frequencies of the radio bursts, and, except for the Sun, Jupiter has the strongest magnetic field in our solar system.

Problem #1: The Ionosphere

Artist’s concept of the magnetosphere. The rounded, bullet-like shape represents the bow shock as the magnetosphere confronts solar winds.Credit: NASA

The upper region of the Earth’s atmosphere, called the "ionosphere," creates a problem when trying to detect very low frequency signals. The density of electrons in this region turns it into a sort of mirror, causing radio signals below 3 MHz to bounce back into space. Incoming radio waves that are below this ionosphere cut-off never reach the ground, and so can’t be observed by ground-based radio telescopes. Even signals that are above this cut-off experience some degradation as they pass through the ionosphere.

Tom Carr, an astronomer who studies low frequency radio emissions at the University of Florida’s Radio Observatory, says that although much of Jupiter’s radio emissions are above the ionosphere cut-off, the signals still can be difficult to detect. Jupiter is, relatively speaking, right next door, so an extrasolar planet many light years away might be even harder to detect. Given that only one planet in our solar system emits frequencies above the ionosphere cut-off, what are the odds of finding higher frequency signals from extrasolar planets?

"The current generation of models predict that a small number of extrasolar planets may have emissions above the ionosphere cut-off," says Joseph Lazio of the Naval Research Laboratory, one of the participants of the VLA planet search. However, "no radio emission from an extrasolar planet has been detected yet."

The LOFAR team has suggested that because many of the known extra-solar planets are much more massive than Jupiter, they also may have larger magnetic fields. This could result in a much larger signal. In addition, many of these planets are very close to their host stars – some with orbits lasting only a few days. Being so close to their stars, these plants probably undergo intense exposure to solar wind. The intensity of the solar wind may serve to increase the power of planetary radio signals.

Problem #2: Planetary Characteristics

In our solar system, a planet’s mass, magnetism, and proximity to a star all factor into the strength of a radio signal. For instance, the Earth, although less massive than Uranus and Neptune, has a brighter radio emission due to its closer orbit around the Sun.

In addition, a planet’s spin rate can influence the radio signal, as can the presence of moons. Jupiter’s radio emission, for instance, is driven by the orbital energy of the innermost moon Io. Jupiter’s rotational energy and energy storage in the magnetosphere also affect the strength of its radio signal.

We don’t know if these factors hold true for planets outside our solar system, however. Lazio notes that many of the extrasolar planets may be tidally locked to their stars. This means the planets would have a slow spin rate, and therefore may have a weak magnetic field and a weak radio signal.

We don’t even know if extrasolar planets have magnetospheres – they may instead be non-magnetized bodies like Mars or Venus. Or they could be like Mercury, a planet with a weak magnetosphere and no significant atmosphere. Auroral activity requires an ionized upper atmosphere, so Mercury doesn’t emit Aurora-related radio.

Dennis Gallagher, a plasma physicist at NASA’s Marshall Space and Rocket Center, suggests that younger planetary systems will be the most promising sources of radio bursts.Credit: NASA

Dennis Gallagher, a plasma physicist at NASA’s Marshall Space and Rocket Center, says that older planetary systems, where the electromagnetic dynamo responsible for planetary magnetic fields has slowed down, may be less likely to have Aurora-related radio bursts. He suggests that younger planetary systems will be the most promising sources of radio bursts.

"It would seem likely that any relatively young system will have planets with either liquid cores or gas giants with fluid cores," says Gallagher. "It’s hard to imagine a planet forming without rotation, and the combination appears likely to create a magnetic field. If you have a magnetic field and active sun, then you just about have to have radio emissions."

According to Gallagher, our ability to detect will be influenced by how the signal is sent out into space. The radio bursts are composed of accelerated beams of energized particles, and it would be easier for us to find the beams when they are directed towards Earth.

"Some emissions are both strong and directed," says Gallagher. "Any directed beaming would increase the signal strength when the signal passes the Earth."

Problem #3: Solar Radio

Stars also emit low frequency radio waves. Our Sun, for instance, emits various frequencies often in the same range as Jupiter, depending on the amount of solar activity. As radio astronomers point their telescopes to tiny pinpricks of starlight, how will they be able to discriminate between planetary and stellar radio frequencies?

Because Aurora radio bursts are dependent on the Sun-planet relationship, planetary radio bursts may be coordinated with the planet’s orbit around its star. This would give the planets a signature radio signal that is quite different from the typically sporadic radio bursts of a star.

Problem #4: Noise

The constant galactic background radiation adds another complication to radio planet searches. In addition, terrestrial radio interference at low frequencies is an increasingly serious problem for radio astronomers. This huge amount of radio noise that we generate on Earth will make things difficult for LOFAR.

"The concept of searching for magnetospheric planets this way has been looked at before," says Gallagher. "There was a proposal once to locate a Very Long Baseline Interferometer on the far side of the moon in order to avoid local radio noise."

Building an array on the moon is both financially and technically unfeasible at the moment. The current, more practical sites under consideration for LOFAR are in the southwestern United States, western Australia, and the Netherlands.

Because of noise interference, the further away an extrasolar planet is from our solar system, the more difficult it will be to detect its radio emissions.

"Even at the distance to the nearest star – Proxima Centauri, 1 parsec (3.258 light years) away – none of the planets in our own solar system would be detectable using the LOFAR array," notes Mutel.

Hunting in the Dark

The image above is an artist’s conception of one LOFAR station, made up of more than 100 dual-polarization antenna systems. LOFAR will consist of ~100 such stations covering an area ~400 km in diameter at a site yet to be chosen.Credit: lofar.org

Despite the problems facing radio planet detection, it could offer some advantages over current, visual light techniques.

The visual light techniques have never actually seen their quarry: at such great distances, weak planetary light is overwhelmed by the star’s radiance. Planet hunters instead look for the subtle effects an orbiting planet exerts on its host star. Since more massive planets exert more detectable effects, all of the planets discovered so far are gas giants like Jupiter or Saturn. By searching for planetary radio emissions, however, Earth-sized planets possibly could be found. Provided that mass and magnetism are not related, the power of the radio emissions matters more than the mass of the planet.

Another benefit of radio planet searches would be speed. Current detection techniques require astronomers to observe a star for more than one planetary orbit – that’s why most of the extrasolar planets discovered to date are very close to their stars. For a planet orbiting from 1 AU, astronomers need at least two years of observations. Astronomers haven’t been looking for extrasolar planets long enough to discover orbits further away than Jupiter’s. With radio detection, however, planets far away from their stars would be more easily detected. Lazio says that a radio search could do a "quick look" sky survey, detecting planets in just a few hours or days.

Besides just finding the extrasolar planets, radio emissions could tell us about a planet’s magnetic field, the rotation rate, the spin axis, if there are any moons, and whether a planet is rocky or gaseous.

Gallagher says extrasolar radio signals also would allow us to compare other solar systems with our own. Does solar wind from similar stars behave the same? Is there is a strong relationship between a planet’s mass, age, and magnetic field properties? Such questions could start to be answered by comparing Aurora-related radio bursts.

What’s Next

The project scientists currently are using the VLA to try to detect radio emissions from confirmed extrasolar planets. In the next 5 years, the scientists hope to use LOFAR to discover previously unknown planets.

Once built, LOFAR will operate in the 10 to 250 MHz range. The radio telescope will do more than hunt for extrasolar planets. LOFAR will be able to study such things as the Earth’s ionosphere, Coronal Mass Ejections of the Sun, and the most distant galaxies and quasars.